The present invention relates to an overvoltage arrester for high or medium voltage having an arrester block arranged inside a sealed, gas-tight enclosure housing.
An overvoltage arrester of this kind is described in heretofore, e.g., A2 European Patent Application No. 0 388 779 A2.
When an arrester having no spark gap is in its standby state, leakage current flows through the non-linear resistor elements, causing the body of the arrester to heat up slightly. As the arrester gets older, this leakage current may slowly increase, thus raising the average temperature of the arrester.
By measuring the temperature increase in the arrester having no spark gap, one can monitor the extent to which it has aged. Moreover, in arresters having a spark gap, by measuring the temperature one can draw certain conclusions about processes in the arrester. In addition, it is useful to obtain information about further operating variables of the arrester, these being determined inside the enclosure housing.
An the object of the present invention is to provide an overvoltage arrester whose working condition and the extent to which it has aged, e.g., temperature, current, gas pressure or gas humidity, can be easily and conveniently monitored, and a method that allows one to reliably monitor the arrester and draw conclusions about its condition.
According to the present invention, this object is achieved as follows: A sensor, in particular a temperature sensor in the form of a surface wave sensor is arranged inside the enclosure housing and integrated into the arrester block.
A radio-queriable surface wave sensor is a passive, acoustic strip element to which a query signal in the form of an electromagnetic wave can be radiated from outside the arrester via an antenna, this signal being received via an antenna, radiated back in modified form based on certain physical values, e.g., the ambient temperature around the surface wave sensor, and picked up again by an antenna outside the enclosure housing. Thus, the measured value for the measured variable, in particular the temperature inside the enclosure housing of the overvoltage arrester, is made available for further processing to a query device outside the enclosure housing arranged, for example, at the foot of the arrester, further measures being unnecessary, and can, for example, be forwarded to a central data processing station via fiber optic cable, radio or other measuring line.
Furthermore, the signals radiated back by various different surface wave sensors may be encoded by the individual surface wave sensors, so that the signals of closely adjacent overvoltage arresters can easily be distinguished from one another and assigned accordingly. Moreover, the behavior of a surface wave sensor may be changed irreversibly if the sensor is temporarily overloaded. Thus, an overload that has occurred in the past can be determined from the altered behavior of the surface wave sensor. This feature can be used to record arrester overloads or total failures.
In normal cases, a discharge current flows for a very short time, so that a large amount of energy is converted into heat in the arrester block in a very short time. As a result, for a short time the arrester heats up significantly, which is reflected in a temperature jump that can be recorded by the surface wave sensor. The energy converted in the arrester can be calculated from the temperature difference associated with a temperature jump of this kind multiplied by the mean heat capacity of the arrester material and, respectively, from the appropriate calibration curve, and, respectively, the discharge processes can be counted so that the condition of the arrester can be documented or maintenance work performed.
To accomplish this, according to the method according to the present invention, if the temperature of the arrester block jumps suddenly, the electrical energy converted in the arrester may be determined from the temperature difference and the heat capacity.
The temperature values may be recorded on an ongoing basis by the surface wave sensor. In this case a stationary query unit radiates signals to the surface wave sensor on an ongoing basis and receives and evaluates the signals that are radiated back.
Alternatively, the individual surface wave sensors of a group of arresters may be queried using a portable query device only when maintenance is required, or periodically.
According to an advantageous embodiment of the overvoltage arrester according to the present invention, the surface wave sensor is arranged inside an at least partly metallic housing, whose walls or other components form an antenna and which is inserted between two discharge elements in the axial direction of the arrester block, or between a discharge element and a connector electrode.
Typically the metallic housing may be designed as a hollow cylinder having caps at both ends, these being made of, for example, aluminum. The metallic housing may then, for example, have at least one longitudinal slot which extends parallel to the longitudinal axis of the arrester body and functions as a slot antenna for receiving and radiating the signals exchanged between the query device and the surface wave sensor. To accomplish this, two connecting leads of the surface wave sensor, which is arranged inside the metallic housing, are conductively connected to this housing.
The metallic housing or a part thereof may also be designed as a patch antenna that includes two conductive layers having a dielectric layer arranged between them. Such slot antennas, patch antennas or micro-strip antennas of this kind are known heretofore according to, for example, Meinke, Grundlach: Taschenbuch der Hochfrequenztechnik (The High-frequency Technology Pocket Handbook), 5th edition, Springer Verlag, Berlin, Heidelberg, New York, and according to the journal article Input Impedance and Radiation Pattern of Cylindrical-Rectangular and Wraparound Microstrip Antennas, IEEE Transactions on Antennas and Propagation, Vol. 38, No. 5, May 1990.
Furthermore, it is useful if the housing conducts the discharge current if a discharge event occurs.
In this case, the current-carrying capacity of the metallic housing must be designed so that the housing can carry the discharge current without the housing or the surface wave sensor being damaged due to overheating.
To this end, the housing may be adhesively bonded to the directly adjacent discharge elements or held in contact with them by the load imparted by spring.
According to a further advantageous embodiment of the present invention, the housing is cylinder-shaped and fits into the outline of the arrester block.
Thanks to this design, high dielectric stability can be achieved, having no protruding edges that could facilitate discharge.
According to a further useful embodiment of the present invention, the surface wave sensor is attached to an inside wall of the housing that is directly adjacent to a discharge element.
As a result, the surface wave sensor takes on the temperature of the adjacent discharge element with no significant delay, so that the temperature indicated accurately reflects the instantaneous temperature of the arrester.
The surface wave sensor may be arranged outside the arrester block, in the gas area of the overvoltage arrester, so that the temperature of the overvoltage arrester or some other measured variable such as the gas density or the gas humidity of the filler gas can be monitored. However, the surface wave sensor must be favorably fitted into the antenna in dielectric terms, i.e., so that there is no significant field distortion of the electrical field.